1. Field of the Invention
The present invention relates to a method of and apparatus for reducing the catalyst particulate contamination in flue gas and main column bottom liquids in a fluidic catalytic cracking (FCC) system without resort to tertiary catalyst recovery equipment. More particularly, the present invention relates to an improved method of and apparatus for withdrawing extremely small catalyst particles from the catalyst inventory in an FCC system.
2. Discussion of the Prior Art
The field of catalytic cracking, particularly fluid catalytic cracking, has undergone significant development improvements due primarily to advances in catalyst technology and product distribution obtained therefrom. With the advent of high activity catalysts and particularly crystalline zeolite cracking catalysts, new areas of operating technology have been encountered, requiring refinements in processing techniques to take advantage of the high catalyst activity, selectivity and operating sensitivity.
By way of background, the hydrocarbon conversion catalyst usually employed in an FCC installation is preferably a high activity crystalline zeolite catalyst of a fluidizable particle size. The catalyst is transferred in suspended or dispersed phase condition with a hydrocarbon feed generally upwardly through one or more riser conversion zones (FCC cracking zones), providing a hydrocarbon residence time in each conversion zone in the range of 0.5 to about 10 seconds, and usually less than about 8 seconds. High temperature riser hydrocarbon conversions, occurring at temperatures of at least 1000.degree. F. or higher and at 0.5 to 4 seconds hydrocarbon residence time in contact with the catalyst in the riser, are desirable for some operations before initiating separation of vaporous hydrocarbon product materials from the catalyst.
Rapid separation of catalyst from hydrocarbons discharged from a riser conversion zone is particularly desirable for restricting hydrocarbon conversion time. During the hydrocarbon conversion step, carbonaceous deposits accumulate on the catalyst particles and the particles entrain hydrocarbon vapors upon removal from the hydrocarbon conversion zone. The entrained hydrocarbons are subjected to further contact with the catalyst until they are removed from the catalyst by a separator, which could be a mechanical means, and/or stripping gas in a separate catalyst stripping zone. Hydrocarbon conversion products separated from and materials stripped from the catalyst are combined and passed to a product fractionation step. Stripped catalyst containing deactivating amounts of carbonaceous material, hereinafter referred to as coke, is then passed to a catalyst regeneration operation.
Movement of catalyst particles through the riser conversion zone, through various inertial and cyclone separators, through catalyst stripper baffles and through the catalyst regenerator, causes substantial catalyst particle breakage and over time will reduce the average size of catalyst particles in a dense bed storage area inventory. As is well known, the smaller the particle size, the more easily entrained that particle is in an airflow of a given velocity, and the particle can be carried by either gaseous hydrocarbon effluent passing from the reactor vessel to the fractionator or by flue gas travelling from the catalyst regenerator to the atmosphere.
By reference to FIG. 1, a typical FCC system is illustrated together with various known tertiary catalyst recovery systems. The hydrocarbon reactor feed is supplied to an FCC riser conversion zone 10a in a reactor vessel 10 along with regenerated catalyst from regenerator 12 and new catalyst from catalyst replenish 11, whereupon it travels through the riser conversion zone as previously noted and the hydrocarbons are catalytically cracked as usual. Using separator 10b, (which could be either riser cyclones or inertial separators), and then secondary cyclones, all contained within the reactor vessel 10, catalyst particles are separated from the cracked hydrocarbon effluent and these catalyst particles pass to a dense bed storage area in the lower portion of the reactor vessel 10. There may be stripping stations located in the lower portion of reactor vessel 10, where steam is passed through the separated catalyst in order to remove as much of the entained and/or entrapped hydrocarbon materials from the catalysts as is possible. Then the catalyst is returned to a catalyst regenerator 12, where it is mixed with air and heated until hydrocarbon impurities remaining in and on the catalyst are burned off leaving regenerated catalyst. The gases from the burning process are passed through one or more cyclone separators, where the catalyst particulate matter is removed and the exhaust gases are passed into the atmosphere by way of stack 14.
Due to catalyst breakage during FCC conversion and regeneration, catalyst "fines" are created in the catalyst inventory which may have particle sizes less than 10 microns in diameter. These particles are very easily entrained in any gas flow and are generally not completely removed during the first stage of separation in the regenerator. Because it is undesirable to permit these particles to pass into the atmosphere through stack 14, several different types of equipment have been used in the past to reduce the amount of catalyst "fines" in the flue gases. An electrostatic precipitator 16 can be placed in the flue gas path through stack 14 and by virtue of charging the catalyst particles, can attract the particles to a catalyst disposal area. Additionally, a third stage cyclone separator 18 can be added in place of or in conjunction with the electrostatic precipitator to further reduce the volume of catalyst "fines" which are transmitted to the stack 14 and from there into the atmosphere.
The catalyst "fines" are also carried by way of the cracked hydrocarbon gaseous effluent leaving reactor vessel 10 into the fractionator main column 20 where they will tend to settle into the lowermost portion of the column contaminating the Main Column Bottom (MCB) products, such as carbon black oil and/or marine diesel fuel, produced therein. Marine diesel fuel specifications generally require no more than 50 ppm of catalyst "fines", and carbon black oil specifications generally require no more than 500 ppm of "fines". Thus, in order to maintain these product specifications, it has been necessary to utilize another or third stage cyclone 22, a liquid electrostatic precipitator 24, a settling tank 26, or a combination of all three, to remove catalyst "fines" from the liquid produced by the fractionator main column 20.
The addition of any tertiary catalyst recovery equipment is an expensive addition to existing systems and comprises a substantial anticipated expense with new refinery systems being built. Further, the movement of the flue gas or gaseous effluent through third stage cyclones and precipitators requires a certain amount of additional energy increasing the cost of refinery products. Furthermore, many refineries are utilizing flue gas expanders to obtain additional energy from the flue gas prior to its release to the atmosphere and the presence of significant quantities of catalyst particulate matter erodes the blades and degrades the performance of turbine expander systems.